Related art heat treatment systems are configured to heat wire rods by applying electric current through wire rods. For example, there is a conventional heat treatment system configured to perform the steps of drawing a wire rod wound in a coiled manner, heating the drawn wire rod to a specified temperature using a direct resistance heating apparatus, cooling the heated wire rod using a cooling apparatus, and rewinding the cooled wire rod in a coiled manner (see, e.g., JP2002-256347A).
The direct resistance heating apparatus is provided to perform quenching and annealing on the wire rod. The direct resistance heating apparatus includes a power supply and two sets of current-applying rolls configured to apply electric current to the drawn wire rod with each set holding the wire rod in a rolling manner. The wire rod passing between the two sets of current-applying rolls is controlled to have a target temperature suitable for heating.
Immediately after the start of heat treatment of the wire rod, or immediately before the end of the heat treatment, the heated temperature of a portion the wire rod passing between the two sets of current-applying rolls does not reach the target temperature, so the portion of the wire rod that has passed between the current-applying rolls immediately before or after reaching the target temperature becomes off-grade.
Therefore, in the conventional example, the system is designed to detect a moving speed of the wire rod in a drawing section using a sensor, and to detect current flowing in a portion of the wire rod passing between the two sets of current-applying rolls using a current sensor, so as to control the power supply such that the passing portion of the wire rod reaches the target temperature, based on signals from the speed sensor and the current sensor.
The wire rod that is heat-treated by this conventional system is relatively thin (about 2 mm), and is wound in a coiled manner after the heat-treatment.
With regard to a relatively thick wire rod of about 7 mm to about 19 mm, there are demands for processing (e.g., pressing) the wire rod in-line after the heat treatment. However, the conventional system cannot handle such processing after the heat treatment, as the wire rod is wound in a coiled manner directly after the heat treatment.